Lithium batteries (i.e., batteries containing metallic lithium or a metallic lithium alloy as sole electrochemically active negative electrode material) are becoming increasingly popular as portable power sources for electronic devices having high power operating requirements. Among these lithium batteries, lithium-iron disulfide (i.e., batteries utilizing iron disulfide as the primary electrochemically active positive electrode material) batteries are the most widely used and prevalent 1.5 volt consumer battery sizes (i.e., AA and AAA).
Lithium-iron disulfide batteries are usually made from pyrite, a naturally-occurring ore that contains mostly iron disulfide (preferably, in excess of about 90 wt. % and more preferably at or above about 95 wt. %). The natural ore is crushed, heat treated, and dry milled (e.g., jet milled) to a mean diameter of the volume distribution that is between 20 to 30 microns. In this approach, the fineness of the grind is limited by relatively hardness of the mineral and the reactivity of the particles with air and moisture. As the particle size is reduced, the surface area increases and becomes more susceptible to weathering, which is an unwanted oxidation in the presence of moisture and/or air to create iron sulfates. In turn, sulfates may increase acidity and reduce electrochemical activity of the pyrite and, by extension, the final cathode material. Iron disulfide particles under this approach can have particles sizes that are close to the final cathode coating thickness of about 80 microns because of inconsistencies in the dry milling process, and large particle sizes can negatively impact processes such as compaction/calendaring (causing substrate distortion), coating to substrate bond disruption and separator damage.
Under an improved manufacturing approach, the pyrite particles from natural ore may be milled in situ within the coating slurry through the use of a media mill. This approach can yield much smaller particle sizes and avoids the weathering issue, but the in situ nature of the media milling restricts the manufacturer to a single particle size profile based upon the resident time in which the slurry is subjected to milling. Stated differently, in a media milling operation, it is difficult to tailor the shape of the particle distribution produced by the milling, which is usually expected to be fairly uniform.
Pyrite particles derived from natural ores also contain a number of impurities. In particular, natural pyrite typically contains metal-based impurities containing metals such as Si, Mn, Al, Ca, Cu, Zn, As, and Co. Impurities are believed to decrease theoretical capacity input and contribute to problems such as shorting and/or other problems. The total concentration of various impurities in natural pyrite ore varies based upon mining and storage conditions, although they are often at least about 3 wt. % of the overall material. Notably, although pyrite and iron disulfide may be used interchangeably herein, a portion of that material may include non-electrochemically active constituents depending upon the context, and any reference to purity of pyrite or iron disulfide should be understood and interpreted accordingly.
Synthetic pyrite is also available as a potential raw material. Owing to the synthesis process, these materials can be substantially more expense than natural ore and the mean diameter of the volume distribution for synthetic pyrite typically has an average particle size anywhere from tens of nanometers up to about 2 microns. While synthetic pyrite can be produced with little or no metal-based impurities as found in natural pyrite, some synthetic pyrites may contain iron sulfides having forms other than FeS2. For example, some types of synthetic pyrite may also contain iron sulfide (FeS), marcasite (a distinct and less preferred form iron disulfide) and/or lesser order iron sulfides such as pyrrhotite (FeS1.3), all of which may have unwanted and/or unpredictable effects upon the electrochemical performance of synthetic pyrite. Additionally, synthetic pyrite has been observed to undergo greater volumetric expansion upon discharge as compared to natural pyrite. Finally, synthetic pyrite present challenges owing to the pyrophoric nature of extremely small particles.
The discharge reaction between lithium and iron disulfide is unique in comparison to the class of cathode compounds normally considered as candidates for primary lithium batteries. First, as iron disulfide discharges, the commonly accepted lithium-iron disulfide electrochemical reaction is expressed as 4 Li+FeS2→2 Li2S+Fe. However, the inventors have determined the proposed reaction mechanism involves at least two distinct reactions, including and the formation of an intermediate phase that ultimately concludes with a complete displacement reaction:2Li+FeS2→Li2FeS2 2Li+Li2FeS2→2Li2S+Fe
Second, iron disulfide cathodes undergo significant volumetric expansion during discharge in comparison to other cathode materials. In fact, as described in United States Patent Publication 2009/0104520 (incorporated by reference), iron disulfide cells and cathode coatings both must have sufficient amounts of void engineered into the cell design in order to avoid physically compromising the battery container. United States Patent Publication 2010/0273036 (also incorporated by reference) goes on to further suggest that even when the container has sufficient strength and the cell/cathode design has sufficient void, the expansion experienced by the cathode is non-uniform and causes deformation in parts of the coated iron disulfide cathode that can lead to physical penetration of the separator layer adjacent to the cathode.
One reason for the non-uniformity of expansion may relate to the fact that, unlike other electrode materials, the shape and morphology of pyrite varies from particle to particle and is not consistent. Depending upon the source (natural ore vs. synthetic) and the conditions under which the material was extracted and stored, pyrite may have a smooth or rough morphology, or a mixture of both. Moreover, the shape of the particles is rarely ever spherical and instead encompasses any number of polygonal cross-sectional shapes.
In the past, only a single pyrite source having a consistent composition was used, in part to avoid discontinuities in expansion upon discharge, difficulties with rheology/mix processing and/or unwanted variability in performance of the pyrite as a cathode material.